A normal uterus does not contract vigorously throughout most of pregnancy and thus provides a tranquil environment for the growing fetus. At term, the myometrium (muscular tissue of the uterus) undergoes a series of changes that lead to synchronous, rhythmic uterine contractions, that is, labor. Contractions of the uterus are directly proportional to the underlying electrical activity of the muscle. The frequency, duration and magnitude of a uterine contraction are directly proportional respectively to the frequency, duration and propagation of action potentials in the myometrium and other muscle cells associated with movement of the uterus. (A similar situation exists in heart muscle although heart and uterine muscle are different with respect to structure and configuration of the action potentials.) Between bursts of action potentials, the uterus relaxes and recovers. The relaxation phase in uterus, although perhaps not as critical as in the heart where refilling must occur, is still very important in providing a respite for both the muscle and the fetus.
By recording uterine electrical activity, one can assess the contractility of the myometrium. It is known to record uterine myometrial electrical activity using electromyography (EMG) wherein electrodes are placed directly on the uterus. As another example, U.S. Pat. No. 5,776,073 to Garfield et al. describes a method and apparatus for recording uterine electrical activity from the surface of the abdomen or vagina for the purpose of diagnosing contractile patterns of the uterus or abdominal muscles in pregnant and nonpregnant patients.